High pressure electric discharge lamps containing Hg and rare earth iodides are commercially available and used for studio lighting. These sources have high efficacy, greater than 80 LPW, good color rendering, CRI approx. equal to 85, and a high color temperature, approx. 6000.degree. K. The high color temperature is compatible with photographic film. Sources for more general illumination should have the high efficacy and good color rendering of the rare earth studio lamps, but a warm color temperature, approximately 3,000.degree. K., more representative of an incandescent source, would be desirable.
The high efficacy and good color rendering of rare earth halide lamps arises from both atomic and molecular emission from the arc. Many rare earth atomic emission lines in the visible region of the spectrum originate from the central core of the arc. Superimposed on the atomic emission spectrum is molecular emission from the rare earth subhalides, which comes from the mantle of the arc. Since the radiation from the rare earth halide sources is deficient in the red, compared to the blue and green, a high color temperature results.
One approach to lowering the color temperature is the addition of alkali atoms, such as sodium or lithium. These are added as the iodides to reduce reaction with the lamp envelope. The discharge typically contains cesium iodide to help broaden and stabilize the arc, and provide a source of atoms with low ionization potential (cesium ionization potential=3.9 eV). Ionized cesium provides the electrons necessary for maintaining the discharge and reduces the cesium neutral emission in the IR which lowers the efficacy of the lamp. Ionization of cesium also lowers the extent of ionization of the rare earth atoms. This is desirable because maximization of rare earth neutral atoms increases the visible emissions. Addition of sodium alone lowers the color temperature and increases the efficacy, but at the expense of color rendering. The sodium emission is predominantly located at 590 nm and tends to dominate the spectrum. Also, addition of the sodium can increase the rare earth ion to neutral ratio because of the higher ionization potential of sodium relative to cesium. Addition of lithium results in emission at 671 nm. Although emission from this line lowers the color temperature, the emission is far outside the photopic response, and efficacy decreases.